Shift control system for a hydromechanical tractor transmission

ABSTRACT

A shift control system for automatically shifting a multi-speed hydromechanical transmission one gear ratio at a time. The shift control system includes a cam element and an electronically controlled shifter assembly. The cam element is shiftable between distinct angular positions corresponding to different and distinct gear ratios for the transmission. The shifter assembly includes an actuator for imparting movement to the control element and a pair of stop assemblies. The stop assemblies limit angular movement of the cam element in incremental steps and, thereby, sequence transmission operation to one gear ratio at a time.

FIELD OF THE INVENTION

The present invention relates to hydromechanical multi-speed tractortransmissions having a plurality of hydraulic powershift clutches and,more particularly, to a control system for automatically andsequentially shifting such a hydromechanical tractor transmission onegear ratio at a time.

BACKGROUND OF THE INVENTION

Hydromechanical transmissions for tractors and the like typicallyinclude a plurality of gear ratios for providing a multitude of groundspeeds for the tractor. Some tractors provide as many as 24 differentforward speeds. Although 24 different speeds are provided, such atransmission develops relatively slow ground speeds with high torque. Asan example, and although 24 different speeds are provided, the averagedifference in ground speed for the tractor between successive gearratios is less than two miles-per-hour.

Such transmissions typically include several powershift clutches whichare selectively operated to provide a respective one of thetransmission's multi-gear ratios. Unlike other clutch mechanisms whichoperate in response to manipulation of a clutch pedal to disengage thetransmission from a power source attendant to each shift or change ingear ratios, powershift clutches are typically hydraulically actuatedclutch mechanisms which operate in response to pressurized fluid beingdirected thereto. The flow of pressurized fluid is directed to thevarious powershift clutches as a function of the position of an operatorcontrolled shift lever.

The powershift clutches are arranged in the transmission such that asequence of successively higher gear ratios, from neutral to an Nthspeed, are obtainable as the shift lever is moved by the operatorwithout disengaging the transmission from the power source. As such,each different position of the shift lever conditions the transmissionto operate in a respective one of its multiple gear ratios. Each gearratio corresponds to a different tractor speed. Naturally, efficientoperation of the tractor is promoted by convenient transmission gearratio selection.

As is known, rapid shifting of the transmission may result in roughshifts which can jolt the tractor, particularly if a shift is made underload. Because there are so many forward gear ratios and because of therelatively small difference in ground speeds, the tractor operator oftenfinds it difficult to shift the transmission only one gear ratio at atime while progressively increasing the speed of the tractor up to thedesired speed.

Understandably, the tractor operator often wants to reach the desiredground speed as fast as possible. As such, the tractor operator oftentimes moves the shift lever too quickly thus imparting shift shock andits associated drawbacks to the tractor and often times "overshoots" thedesired speed.

SUMMARY OF THE INVENTION

In view of the above, and in accordance with the present invention,there is provided a shift control system for automatically shifting ahydromechanical transmission. Such a shift control mechanism includes acontrol element or cam and an electronically controlled shifter assemblyfor moving the control element such that the transmission is sequencedone gear ratio at a time until the desired gear ratio has been achieved.

In the illustrated embodiment, the control element or cam of the shiftcontrol mechanism is rotatably mounted on the transmission. The controlelement is shiftable between distinct angular positions corresponding todifferent and distinct gear ratios for the transmission. In effect, thetransmission is conditioned to operate in various gear ratios as afunction of the angular position of the control element.

The shifter assembly of the present invention is responsive toelectrical shift signals derived from an electronic control unit.Preferably, the control unit includes a suitably programmedmicroprocessor which receives input signals indicative of a desired gearratio selection and other input signals indicative of operatingperformance of the tractor. The shifter assembly further includes anactuator or driver for imparting angular movement to the control elementand a pair of stop assemblies. The stop assemblies are operated inconcert with the actuator for limiting angular movement of the controlelement in incremental steps.

In accordance with the present invention, the shifter assemblypreferably includes a profiled member having a series of radiallyextending projections thereon. Such a profiled member is driven in timedrelation with the control element such that the spacing between theradial projections on the profiled member is proportional to the angulardistance which the control element rotates to effect a shift in gearratios for the transmission. The stop assemblies act in combination withthe profiled member to limit angular movement of the control element tosequence shifting of the transmission one gear ratio at a time.

By having the control unit operate the shifter assembly in response toshift selection signals and performance characteristics of the tractor,the transmission may be shifted automatically one gear ratio at a time.As such, this shift control system can be used to provide automatictransmission controls for constant load or speed of the tractor whenused for heavy or light load conditions.

Other features and advantages of the present invention will becomereadily apparent from the following detailed description, appendeddrawings, and the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front elevational view of a shift control systemaccording to the present invention;

FIG. 2 is a side elevational view of the shift control system as mountedon a hydromechanical transmission; and

FIG. 3 is a simplified block diagram of an actuating circuit for theshift control system of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

While the present invention is susceptible of embodiment in variousforms, there is shown in the drawings a presently preferred embodimenthereinafter described, with the understanding that the presentdisclosure is to be considered as an exemplification of the invention,and is not intended to limit the invention to the specific embodimentillustrated.

Referring now to the drawings, wherein like reference numerals indicatelike parts throughout the several views, a tractor transmission shiftcontrol system 10 is schematically illustrated in FIG. 1. Asillustrated, the shift control system controls a hydromechanicaltransmission 12 (FIG. 2) of the type disclosed in commonly assignedpatent application Ser. No. 204,167 filed June 8, 1988; the fullteachings of which are incorporated herein by reference.

Suffice it to say, the hydromechanical transmission 12 includes aplurality of hydraulically operated powershift clutch assemblies whichcondition the transmission to operate in various gear ratios dependingupon selective clutch engagement. The clutch assemblies are controlledby various control valves whose disposition within a valve body (notshown) determines which clutch assemblies are operated and, thereby, inwhich gear ratio the transmission is conditioned for operation.

The control system of the present invention is provided to automaticallyregulate operation of such control valves and, thereby, automaticallycontrol the transmission 12. To effect such ends, the control system ofthe present invention includes a control element 14 and anelectronically controlled shifter assembly 16 which is responsive toelectrical shift signals derived from a control unit 18 (FIG. 3).

Control element 14 regulates the operable position of each clutchcontrol valve within the valve body of the transmission. As illustrated,control element 14 includes a cam 20 mounted for rotation about a pin 22or other suitable rotational support. Control element 14 issubstantially similar to a cam actuator disclosed in commonly assignedcopending patent application Ser. No. 190,493 filed May 5, 1988 now U.S.Pat. No. 4,844,127; the full teachings of which are incorporated hereinby reference.

Suffice it to say, cam 20 has a specifically designed cam profile forcontrolling the linear disposition of each clutch control valve, and,thereby operation of the transmission as a function of the angulardisposition of cam 20. In the preferred embodiment, such a cam profileis defined by one or more recessed arcuate tracks 24 which are cast ormachined into the cam 20 at various radii such that they do notintersect with each other.

Shifter assembly 16 is provided to control the angular disposition ofcontrol element 14. As illustrated, shifter assembly 16 includes adriver 28 which imparts positive rotational movement to cam 20 in eitherangular direction. Driver 28 is illustrated as a double acting aircylinder but it should be appreciated that alternative forms for driverwould include a double acting hydraulic cylinder, an electric solenoid,or a vacuum cylinder. Shifter assembly 16 further includes limit stops30 and 32 for controlling the extent of angular rotation of controlelement 14.

For purposes of access and ease of manufacture, shifter assembly 16 ispreferably mounted to the transmission at a location spaced from thecontrol element 14 which it is meant to positively drive. In itspreferred form, shifter assembly 16 further includes a mounting bracket34 which is fixedly secured to the transmission. One end of driver 28 isconnected to the mounting bracket 34. The opposite or operative end ofdriver 28 is connected to a linkage assembly 38.

Linkage assembly 38 transmutes the linear drive movement of driver 28into rotational displacement of control element 14. As illustrated,linkage assembly 38 includes a lever 40 having a stub shaft 42 whichpivotally moves with and mounts lever 40 to bracket 34 intermediate theends of lever 40. One end of lever 40 is articulately connected to theoperative end of driver 28. The opposite end of lever 40 is articulatelyconnected to one end of an elongated link 44. The opposite end of link44 is connected to an actuating pivot arm 46 which radially extends fromthe axis of pivot pin 22. As will be understood, movement of actuatingpivot arm 46 imparts rotational movement to the control element 14.

A sprocket gear 50 is also mounted on pivot shaft 42 for rotation withlever 40. A combination gear 52 having axially aligned sprocket gears 54and 56 is also pivotally mounted on bracket 34 in spaced relation withsprocket gear 50. A suitable force transfer means, such as chain 58,serves to drivingly interconnect sprocket gears 50 and 54.

Combination gear 52 is mounted for pivotal rotation on an eccentric stubshaft 60 which is carried by mounting bracket 34. The eccentricity ofstub shaft 60 allows the tension of chain 58 to be regulated asrequired. As will be appreciated, sprocket gear 56 includes a series ofradially extending teeth 62. The drive ratio between sprocket gears 50and 54 is such that one circular pitch of gear 56 equals one gear ratioor position change for the transmission.

A support plate 66 is also secured to mounting bracket 34 by a fastener68. Limits stops 30 and 32 are mounted on support plate 66. Since thelimit stops are mirror images of each other, any description hereafterof one limit stop and the components associated therewith will alsopertain to the other limit stop.

Each limit stop assembly includes a driver 70. Driver 70 is preferablyan air actuated cylinder but it will be appreciated that a hydrauliccylinder, electric solenoid, or vacuum operated cylinder will sufficefor purposes hereinafter described. One end of driver 70 is pivotallysecured to support plate 66. The opposite or operative end of driver 70is provided with a clevis 72 having a laterally extending locating pin74 fixed at the free end thereof. As best seen in FIG. 2, clevis 72includes laterally spaced depending arms 75 and 77 which are connectedto each other and to the operative end of driver 70. Arms 75 and 77 arerespectively disposed on opposite sides of gear 56. Locating pin ormember 74 spans the distance between arms 75 and 77.

Each clevis 72 is accommodated or entrapped between a pair of stops 76and 78. Stops 76 and 78 are secured to support plate 66 and limit theextent of pivotal movement of clevis 72 and driver 70. A spring and pinarrangement 80 normally urges the clevis 72 toward one of the stops 76,78.

In accordance with the present invention, and as illustrated in FIG. 3,the shift control system of the present invention further includes asuitably programmed electrical control unit 18 which preferablycomprises microprocessor circuitry. Microprocessor circuitry ispreferred for its compact size, reliability, versatile program ability,and radial availability at reasonable costs.

Control unit 18 includes electrical circuitry which is programmed toprovide sequential operation of drivers 28 and 70 for effectingtransmission shifting responsively to shift signal and other inputs froma plurality of sources.

Control unit 18 receives shift signal inputs from an operator controlledgear ratio selecting mechanism 82. Control unit 18 also receives othersignal inputs indicative of engine load, tractor speed, and etc. whichare calculated by the control unit and effect transmission shifting.Control unit 18 operates the drivers 28 and 70 in a manner sequencingthe transmission one gear ratio at a time.

In operation, control unit 18 provides output shift signals to thedrivers 28 and 70. The shift signal to driver 28 imparts rotationalmovement to control element 14 in a desired direction. Morespecifically, the linear movement of driver 28 is transmuted intorotational movement of cam 20 through lever 40, link 44, and actuatingpivot arm 46. Because driver 28 positively moves lever 46 in eitherdirection, the transmission gear ratio may be incrementally increased ordecreased positively in both directions.

The extent of angular rotation of control element 14 is regulated by theshifter assembly 16. Upon actuation of driver 28, lever 40 is rotated.Sprocket gear 50 and combination gear 52 are likewise rotated with thelever 40. The circular pitch of the radial projections or teeth 62 ongear 56 is such that one circular pitch change past either stop means30, 32 equals one gear ratio change of the transmission.

Besides energizing driver 28, the shift signals from control unit 18,likewise regulates operation of stop means 30, 32. One stop means isoperated to control transmission up shifting one gear ratio at a time.The other stop means is operated to control transmission downshiftingone gear ratio at a time. When driver 70 of either stop means isenergized, the clevis 72 associated therewith is forced downwardlymoving locating pin 74 into engagement with sprocket gear 56. Assprocket gear 56 rotates as a result of rotation of lever 40, pivotalmovement thereof moves the clevis 72 therewith into contact with eitherof the stops 76 or 78 depending upon the direction of rotation of thecombination gear 52 and whether the transmission is being shifted up ordown. The lateral spacing between stops 76 and 78 is sufficient to allowthe cam 20 to index sufficiently to incrementally change thetransmission condition one gear ratio at a time.

Engagement of clevis 72 with either stop 76 or 78 will prevent furtherrotation of the combination gear and prevent further indexing of thecontrol element 14. As such, transmission 12 will be shifted one gearratio and thereafter operated in that particular gear ratio until driver70 receives another shift signal received from control unit 18 in amanner releasing driver 70 and allowing retraction of clevis 72 therebyallowing further indexed movement of the control element 14. Becausesequential operation of the drivers 28 and 70 is automaticallycontrolled as a result of the shift signal inputs provided to thecontrol unit 18, this shift system can be used to provide automatictransmission control for constant load or speed of the tractor when usedfor heavy or light load conditions.

From the foregoing, it will be observed that numerous modifications andvariations can be effected without departing from the true spirit andscope of the novel concept of the present invention. It will beappreciated that the present disclosure is intended as anexemplification of the invention, and is not intended to limit theinvention to the specific embodiment illustrated. The disclosure isintended to cover by the appended claims all such modifications as fallwithin the scope of the claims.

What is claimed is:
 1. A shift control system for a hydromechanicaltractor transmission which is shiftable between a plurality of differentgear ratios, said shift control system comprising:a cam mounted on saidtransmission for rotation about a pivot and between a series of distinctangular positions, said cam defining a cam profile which controls inwhich gear ratio said transmission is operated as a function of theangular disposition of said cam; and an electronically controlled camshifter assembly which is responsive to electric shift signals derivedfrom a control unit, said cam shifter assembly comprising actuatingmeans responsive to said electric shift signals and including a driverand connecting means between the driver and said cam for rotatablyshifting the cam and imparting angular movement thereto, a profiledmember driven in timed relation to said cam and having a series ofspaced radial projections thereon, the spacing between said projectionsbeing equal to a distance the cam must be rotated to effect a change ingear ratios, and stop means operated in response to said electric shiftsignals and in timed concert with said actuating means for limitingangular movement of said cam in incremental steps.
 2. The shift controlsystem of claim 1 wherein said stop means act in combination with saidprofiled member to limit angular movement of said cam.
 3. The shiftcontrol system of claim 1 wherein said stop means includes a driverhaving one end pivotally mounted and a free end which acts against theradial projections on said profiled member.
 4. The shift control systemof claim 3 wherein said free end of the stop means driver is entrappedbetween a pair of laterally spaced stops.
 5. A shift control system forautomatically shifting a hydromechanical transmission through a seriesof different gear ratios, said shift control system comprising:arotatably driven control element mounted on said transmission andshiftable between distinct angular positions in a manner conditioningthe transmission to operate in different gear ratios depending upon theangular position of said control element; actuating means for impartingrotational movement to said control element, said actuating meansincluding a driver connected to said control element through a linkageassembly; means for limiting rotational movement to said control elementby acting against a profiled member in a locking manner controlling theangular position of said control element, said profiled member beingdriven in timed relation with said control element and defines a seriesof spaced radial teeth, the spacing between said teeth beingproportional to distinct angular positions of said control element, saidlimiting means further includes a pair of drivers each of which includesa member which is movable toward and engagable with said teeth on saidprofiled member and is accommodated between a pair of laterally spacedstops, with one driver of said limiting means acting to control theangular position of said control element during transmission up-shiftsand the other driver of said limiting means acting to control theangular position of said control element during transmissiondown-shifts; and control circuitry means responsive to input signalsfrom an operator influenced shifter for operating said actuating meansand said limiting means in a manner automatically controlling theposition of said control element and thereby transmission operation.